FOLDED DIELECTRIC FILM ELEMENT AND METHOD FOR MANUFACTURING THE SAME

Abstract: A folded dielectric film element functioning as an energy converter, consisting of a dielectric film (1, 2) bent into folds (L) and provided with electrodes (K). The electrodes (K) are so arranged in the folds (L), different electrodes being placed in successive folds, that the strength of the electric field increases in one fold and decreases in the other. The elements of the invention allow a high acoustic power to be achieved. In noise damping aplications, the elements function in a multi-effect manner and they also have a passive damping effect as the air flows between the folds.

Description: FOLDED DIELECTRIC FILM ELEMENT AND METHOD FOR MANUFACTURING THE SAME. The present invention relates to a folded dielectric film element acting as an energy converter as defined in claim 1 and to a procedure for manufacturing such a film element. This type of films can be used for the generation, measure¬ ment and damping of sound. Broadly speaking, the invention is concerned with the conversion of electric energy into mechanical energy or conversion of mechanical energy into electric energy. A film like this is presented e.g. in US patent specification 4 654 546. To permit an electric field to be utilized in this type of energy converters, the air gaps used must be very small to achieve a sufficient field strength. However, this means that the play of the converter is reduced. The object of the present invention is to produce a new solution for increasing the amount of air or medium replaced by the converter by using a folded dielectric film and placing the electrodes in the folds. The features charac¬ terizing the solution of the invention are presented in detail in the attached claims. The solution of the invention makes it possible to utilize increased air motion amounts while simultaneously producing large air pressures, e.g. sound pressures in acoustic applications. The invention also concerns a procedure for the manufacture of converter elements, in which a folded element is first compressed into a tight pack, causing the corners of the folds to be pressed into sharp edges. After this, by stretching the element, the folds assume their natural curvature, resembling a low-gradient letter S, which keeps the folds in a stable position when subject to the forces of the electric field. Elements manufactured according to the invention allow a high acoustic power to be reached. In noise damping applica¬ tions, the elements function in a multi-effect manner, damping the noise passively while the air is flowing in the gaps between the folds. The electric energy generated by the sound waves is converted into heat in the resistances of the electrodes. A measured signal corresponding to the sound pressure can be fed in opposite phase to another element. The two first-mentioned prinicples damp sound waves of any frequency coming from any direction, and in the case of low frequencies, also those incident at an oblique angle. The sound damping capacity of the element is 20 dB at best. In the following, the invention is described by the aid of an example by referring to the attached drawings, in which Fig. 1a and 1b present a folded dielectric film according to the invention, Fig. 2a and 2b another folded dielectric film according to the invention and Fig. 3a and 3b a third folded dielectric film according to the invention. Fig. 4a presents a variable-gain amplifier circuit and Fig. 4b a curved film. Fig. 5a - 5c illustrate a wall structure and its damping capacity, Fig. 6 and 7 present applications of circuits connected according to the invention and Fig. 8 an electret film with a metal plating on one side. Fig. la presents an embodiment of the invention, showing a densely folded (folds L) dielectric film element consisting of two film layers 1,2 placed one upon the other. In Fig. 1, the electric field is alternately strengthened and weakened in successive folds L by means of a control voltage. In the folded element, the film, which is charged to charge Q, is coated in the successive folds with electrode layers K such that control electrodes of opposite sign are placed on opposite sides of the fold L. Such a film element can be manufactured e.g. from a permanently charged electret film or a three-layer film in which the middle layer consists of semiconducting material and is connected to a d.c. potential (control signal Δ on one side of the layer K and control signal -Δ in opposite phase on the opposite side of the layer K) producing a charge Q. Fig. 1b illustrates the structure of the control electrodes K. Both consist of a rectangular edge and vertical bars starting from it and placed between the bars of the opposite electrode. Each fold L is joined at its ends with an adjacent fold L but is apart from adjacent folds at its middle portion. Such an element can be controlled using traditional amplifier equipment in sound reproduction applications. Since the pressure which can be achieved by the electrosta¬ tic principle is = ^εE 2 = +ε(U/s) where ε is the dielectric constant, E is the strength of the electric field, U is the voltage and s is the distance be- tween the electrodes, it is of essential importance for the achievement of a good linearity that the terms dependent on the distance remain as unchangeable as possible, as is the case in folds L designed according to the invention. Sound reproduction devices employing the electrostatic prin¬ ciple generally use preforated stator plates and a thin movable electrode film between them. Since the force of the electric field affects the stator plates as well, undesir¬ able resonances are generated in them. Moreover, the ele- ments have to be large to produce a sufficient power. The sound reproduction device presented in this invention consists of only one folded film structure with electrodes formed on it. In another embodiment (Fig. 2a), the electrodes K are ar¬ ranged by connecting to the sides of the folds L in succes¬ sive order a positive potential, a positive control signal Δ, a negative potential and a control signal -Δ in opposite phase. The electrodes K are connected by joining the same kind of electrodes together at the edge of the film. The voltage to the positive and negative electrodes can be connected via large resistances. The positive and negative charges can also be replaced with a so-called electret charge, which is injected into the dielectric film in a cyclic fashion with positive and negative charges alter¬ nating. The film of the invention can also be used for the measurement of sound or vibration, i.e. electric energy corresponding to the movement of the electrodes is induced in the Δ and -Δ electrodes. The film element shown in Fig. 1a can be manufactured from a thin plastic film with a metal plating on either side, in which the electrodes K are etched by using a technique em¬ ployed in the manufacture of circuit boards. After this, the film is bent into folds L of a height of e.g. 1 - 10 mm with a distance of 0.1 mm between them. In the structure pre- sented in Fig. 2a, consisting of two films, electret charges are placed between the films. The bars K of the electrodes are spaced at larger intervals and the positive and negative potentials are placed between them (Fig. 2b and 2c). Fig. 4a and 4b present an embodiment of the invention in which two folded films 3 are placed crosswise, one of which is fed with a reference signal I via a variable-gain amplifier A and the measurement signal obtained from the other film, corresponding to the sound pressure, is fed into the input of the variable-gain amplifier in opposite phase. In addi- tion to sound reproduction applications, this film element can be used as an active damping element when the control signal is set to zero. The element can be coated with pro¬ tective layers 4, both of which may be e.g. thin films or one may be a thicker plate. It is possible to place several or at least two elements as shown in Fig. 3a on top of each other with protective layers 4 on the outermost surfaces. In Fig. 3b, the voltage to the positive and negative electrodes is connected via a large resistance R1. As for the elec- trodes K, Fig. 3c corresponds to Fig. 2b. The elements of the invention are very light and durable and have a large power capacity. The films 1,2 are preferably produced from plastic films containing flat gas bubbles because it has been established that such films retain their electret charge well and are as light as possible. A film of this type is presented in US patent specification 4654546. In sound source applications, the invention is especially advantageous because the film movement is transverse to the sound direction, which means that the mass inertial forces of the film do not produce a counter moment in the direction of the sound. In noise damping applications, the elements function in a multi-effect manner. The air flowing between the folds L causes a viscose damping, and so does the visco-elastic motion of the films. By using resistive electrode layers, a phase shift can be created in the element and the directional pattern can be widened only horizontally. To produce different directional patterns, the elements may also have a curved shape (Fig. 4b). The invention can also be utilized in many other applica- tions designed to convert electric energy into mechanical motion or vibration, or conversely to convert mechanical motion or vibration into electric energy. In both cases, a very high efficiency is achieved. Fig. 5 presents an application of the invention in which battens 7 are attached to each side of a wall 8 or plate and elements 5 and 6 are placed on the battens, the elements being electrically connected by conductors 9. A noise gener¬ ates a change of charge in element 5. This is fed in oppo- site phase into element 6. If the wall has a passive damping of e.g. 20 dB and the elements have an efficiency of 10%, complete damping in both directions is achieved (Fig. 5b). The efficiency and damping of the elements can be adjusted by varying the + and - voltages or by selecting a suitable electret potential so as to obtain an optimal efficiency and the best damping for each practical application. Also, the folding density can be varied. It is also possible to include an inductance or some other type of passive component in the conductor 9 to adjust the frequency and phase shift characteristics between the elements 5, 6. Moreover, the resonance spaces between the battens 7 can be utilized (Fig. 5c). A larger number of elements 5,6 can be used in an applica¬ tion, and the elements can be interconnected in various ways depending on the use. An active amplifier or filter circuit can also be used. Since the elements 5, 6 have a passive sound damping effect as well and may have a very high effi¬ ciency, it is not always necessary to use an intermediate passive damping layer. Fig. 6 presents an embodiment of the invention in which every third fold L is connected to the output of a control¬ ling amplifier A whereas every second fold is sound-produc¬ ing and connected as a feedback signal to the amplifier A. Fig. 7 presents a corresponding circuit implemented using electret films. The charges are on the surface of the folds L, and the opposite sides are provided with porous protec¬ tive plates 10. Fig. 8 shows a structure in which an elec¬ tret film (folds L) with a continuous metal plating 11 on one side acts as an element both producing and measuring sound. It is obvious to a person skilled in the art that different embodiments of the invention are not restricted to the ex¬ amples described above, but that they may instead be varied within the scope of the following claims.

Claims: CLAIMS 1. Folded dielectric film element (3,5,6) functioning as an energy converter, consisting of at least one dielectric film (1,2) said film (1,2) being bent into folds (L) and provided with electrodes (K), characterized in that the electrodes (K) are so arranged in the folds (L), different electrodes being placed in suc- cessive folds, that the strength of the electric field increases in one fold and decreases in the other. 2. Film element according to claim 1, characterized in that the electrodes (K) are so arranged in the folds (L) that the electrode layers in successive folds occur in the order: d.c. voltage electrode (+), a.c. voltage electrode (Δ), d.c. voltage electrode (-) and a.c. voltage electrode (-Δ) in opposite phase. 3. Film element according to claim 1 , characterized in that electrodes (K) are so arranged in the folds. (L) that the electrode layers in successive folds occur in the order: electret charge (+), a.c. voltage electrode (Δ), electret charge (-) and a.c. voltage electrode (-Δ) in opposite phase or zero. 4. Film element according to claim 1, characterized in that the film (1,2) provided with a charge (Q) is coated in successive folds with electrode layers such that an a.c. voltage electrode (Δ) and an electrode (-Δ) in opposite phase are placed on opposite sides of the folds. 5. Film element according to claim 1, characterized in that the element is coated wiht at least one protective layer (4). 6. Film element according to claim 5, characterized in that the protective layer is a porous protective plate (10). 7. Film element according to claim 1 , characterized in that the film (1,2) is provided with a continuous metal plating on one side. 8. Film element according to claim 1, characterized in that the film (1,2) is a plastic film containing flat gas bubbles. 9. Film element according to claim 1, characterized in that, for the production or damping of sound, at least one of its folded film elements (L) is provided with feedback and has a sound producing component and a sound measuring component, said components having an electric connection or a vari¬ able-gain amplifier (A) between them. 10. Procedure for manufacturing a folded dielectric film element (3,5,6) functioning as an energy converter and consisting of at least one dielectric film (1,2), said film (1,2) being bent into folds (L) and provided with electrodes (K), characterized in that the electrodes (K) are arranged in the folds (L) by placing different electrodes in successive folds so that the strength of the electric field in adjacent folds increases in one fold and decreases in the other, and that the folded element (3,5,6) is compressed into a tight pack, whereupon the element is stretched so that the folds (L) assume a curvature resembling a low-gradient letter S which keeps the folds (L) in a stable position when subject to the forces of the electric field.

Inventors: KIRJAVAINEN KARI, KIRJAVAINEN, KARI
Applicants: KIRJAVAINEN KARI, KIRJAVAINEN, KARI
Application year: 1994
Main classification: H04R19
IPC codes: H01G7, H04R17, H04R19, H04R31